Signal amplifier



H. M. LEWIS SIGNAL AMPLIFIER June 22, 1943.

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ATTORNEY FIG.5

Patented June 22, 1943 SIGNAL A`MPLIFIER Harold M. Lewis. Allenhurst, N. J., assignor to, Hazeltine Corporation, a corporation of Dela- Ware Application octnbei-Vz, 1941, sei-iai No. 413,275 11 claims. (ci. 11s-1713 'Ihe present invention relates to signal amplifiers and, particularly, to signal amplifiers adapt- 4ed to provide a balanced output from an unbalare used and their output circuits are connected to provide a balanced amplifier output. A signal to be amplied is applied to the input circuit of one of the tubes and a portion of the output of that tube is applied to the input vcircuit of the other, the applied portion being such that the signals developed in the outputs of the two tubes are approximately equal in magnitude. An arrangement of this nature requires an initial critical proportioning and adjustment of the circuit' parameters to provide thedesired balanced output,'and any subsequent" change in the values of the circuit parameters, such', for example, as the act of replacing either one of the amplifier tubes, is very likely to result in substantial unbalance of theA output of the amplifier. Moreover, this arrangement has the disadvantage in certain applications that the repeating ratiois.

iixed at a value equal to twice that of the gain of the particular ampliiier tube to which the input signal is applied.

Another form of prior art amplifier of the phase-inverter type comprises a signal amplier tube having an anode-load impedance yand an unby-passed cathode resistor. The desired balanced output of this arrangement is obtained by coupling to the anode and the cathode elements of theampliiier tube. This arrangement does notrequire quite the critical prportioning of the constants of the circuit parameters used as does the prior art phase inverter previously4 mentioned, but has the disadvantage that the -repeating ratio of the amplifier can never exceed unity. There is the further disadvantage in this arrangement that the cathode of the amplifier tube is maintained above ground potential at signal frequencies, a condition which tends to introduce undesired hum where the ampliiier tube is of the indirectly-heated cathode type, due to leakage between the cathode and its heater ele-l ment.

It is an object of the present invention, therefore, to provide a new and improved signal ampliiier of the type adapted to provide a balanced output from an unbalanced input and one which avoids one or more of the disadvantages and limitations of the prior art devices of this nature.

It is an additional object of the invention to provide a signal ampliiier of the type described in which the desired balance ofoutput is effected not only Without the requirement of critical proportioning and adjustment of the circuit parameters, but remains substantially balanced even though the circuit parameters may change during the operation of the amplier.

It is a further object of the invention to provide a signal amplifier of the phase-inverter type having a newand improved form of gain control,

Y whereby the repeating ratio of the ampliiier may be adjusted at will over a wide range of values while at the same time maintaining a high degree of balance of the amplier output.

In accordance with the invention, a signal amplier comprises a pair of vacuum tubes each having input electrodes and an associated input circuit and output electrodes and an associated output circuit. The output circuits are connected to form a balanced ampliiier output circuit. The amplifier includes means for applying to the input circuit of one of the vacuumtubes a signal to be amplified, means for applying to the input circuit of the other of the vacuum tubes a portion of the output of the one vacuum tube, and means' for maintaining the output of the amplitier .balanced over `a range of values of the p0rtion applied to the input circuit of the aforesaid thereof, referenceis had to the following de-v scription 'taken in connection with the accompanying drawing.v and its scope will be pointed..

out in the appended claims. Referring now to the drawing, Flg. 1 is a circuit diagram, partly schematic,'qf a complete carrier-signal receiver embodying `the invention; Fig. 2 is' a simplied circuitv diagram of a signal amplifier embodying the invention and is used as an aid in explaining the operation of the invention; Figs. 3 and 4 are graphs representing certain operating characteristics ofv a signal amplifier embodying the invention; and Fig. 5 isa circuit diagram representing a modified form of the invention. l

Referring now more particularly to Fig. l, there is represented schematically a complete carrier-signal receiver oi a conventional design embodying the present invention in a preferred form. In general, the receiver includes a radiofrequency, ampliiler I having its input circuit connected to an antenna system II, I2 and having its output circuit connected to an Oscillatormodulator I3. Connected in cascade with the oscillator-modulator I3, in the order named, are van intermediate-frequency amplifier Ill of one or more stages, a detector .and automatic volume control supply I5, an Aaudio-frequency signal amplifier I6, more fully described hereinafter, an audio-frequency amplifier I1 of one or more stages, and a sound reproducer I8. An automatic amplication control or A. V. C. circuit is connected between the output circuit of the automatic volume control supply of unit I5 and the input circuits of one or more of the tubes of the radio-frequency ampler I0, the oscillator-modulator I3, and the intermediate-frequency amplifier Il in conventional manner.

It will be understood that the various just described may, with the exception of the signal amplier I6, be of a conventional construction and operation, the details of which are Well known in the art, rendering detailed description thereof unnecessary. Considering briefly the operation of the-receiver as a Whole, and neglecting for the moment the operation of the signal amplier I6, presently to be described, a desireda carrier signal is selected and amplified by the radio-frequency amplifier I0, converted to an intermediate-frequency carrier signalin the oscillator-modulator I3, amplified in the intermediate-frequency yamplifier I4, and detected by the detector of unit I5, thereby to derive the audiofrequency modulation components comprising the audio signal. The audio-frequency signal iS, in turn, ampliiied in the audio-frequency amplifier I6, further amplified in the audio-frequency amplifier I1 and is reproduced by the sound reproducer I8 in a conventional manner.

The automatic amplification control or A. W. C. bias derived from the A. V. C. supply of unit I is effective to control the ampliiication of one or more of the units I0, I3 and I4 in accordance with the amplitude of the received carrier signal to maintain the carrier-signal input to the detector of unit I5.within a relatively narrow range for a Wide range of received signal intensities.

Referring now more particularly to the portion of the system embodying the present invention, there is coupled to the output of the detector of unit I5 a signal amplier I6. The amplier I6 includes a pair of vacuum tubes I9, 20 having input electrodes comprising control grids 2 I 22 and cathodes 23, 24, respectively, and having output electrodes comprising anodes 25and 26, respectively. The input electrodes of vacuumv tube I9 have an associated input circuit comprising a grid leak 28, and an unby-passed cathode bias resistor 29. 'Ihe input electrodes of vacuum tube 20 also have an associated input circuit compris- Ving a grid leakresistor 30 and the cathode resistor 29. The anode 25,0f tube I9 has an associated output circuit comprising a load resistor 21 and a source of space current comprisinga battery 3I. The anode 26 of vacuum tube 29 similarly has an associated output circuit comprising aload resistorv 32 and the same source `of space current 3|. As thus arranged, the load resistors 21 and 32 are connected in series between the anodes 25 and 26 with their junction grounded at signal frequencies through'source 3l to form a balanced amplier output circuit. A balancing impedance comprising a resistor 33 of relatively large value is connected in series with the resistor 29 between the cathodes of vacuum tubes I9 and 29 and ground. The amplifier I6 includes means for applying to the input circuit of one of the vacuum tubes a signal to be ampliiied, this means comprising a coupling condenser 34 connected between the grid 2| of the vacuum tube I9 and the ungrounded input terminal .1." of the amplifier input terminals :c

There is also included means for applying to the input circuit of the other vacuum tube 29 a portion of the output of the rst vacuum tube I9, this means comprising a coupling condenser 35 connected between vthe grid 22 of vacuum tube 29 and :in/adjustable tap'36 provided on the load resistor 21 included in the output circuit of vac- -uum tube I9.

The signal which is applied from the output circuit of the detector of unit I5 to the input terminals, .1:,of the amplifier I6`is applied between the control grid 2| of this tube and ground and'thus is applied across the resistors 29 and 33.

l that the resistor 33 is eilectively common to the input and output circuits of both of the vacuuin tubes I9 and 29.

tube I9. The signal potential developedacross 'the resistors 29 and 33 and a portion of that developed across the lower portion of the load resistor 21 below the tap 36 are'applied to the input circuit of vacuum tube 20 to develop across the load resistor 32 in the output circuit of this tube.

an ,amplied signal potential. Since the space current of vacuum tube 29 also ows through the resistors 29 and 33, the signal potential developed across these resistors by the space current of 'vacuum tube 26 is of degenerative phase with respect to the input circuit of this tube but is of regenerative phase with respect tothe input circuit of vacuum tube I9. For the special case where the tap 36 is so adjusted that the portion of the ampliiied signal applied from the output of vacuum'tubel I9 to the input circuit of vacuum tube 20 is just equal to that applied from the detector of unit I5 to the input circuit of vacuum tube I9, the signal potential developed across the resistor 33 by the output current of tube I9 is just equal in magnitude and opposite in phase to that developed across this resistor'by the output current of tube 20. Y Consequently', the two signal potentials developed across the resistor 33 eiiectively cancel each other and thus are neither degenerative nor regenerative in the input circuits of tubes erative eiect, as the case may be, in the input circuits of each of the tubes tending to reestablish equality of magnitude and oppositeness of phase between the signal potentials. The reslstor 33, which is`eiectively common to all of the input and output circuits of tubes I9 and 20, has a relatively large value proportioned in a manner presently to be considered in greater detail. It is thus eiective by virtue of the large signal potentials developed thereacross to maintain the output of amplifier I6 balanced over a comparatively wide range of values of the portion of the signal output of Vacuum tube I9, determined by the setting of the adjustable tap 36 on the load resistor 21, which tap selectively controls the value of the above-mentioned portion of the signal output which is applied to the input circuit of the vacuum tube 20 to control the repeating ratio of amplier I6.

The more detailed operation of the amplier I6 will become evident from the following mathematical analysis. The simplied circuit. diagram of Fig. 2 corresponds tothe amplier I6 of Fig. l, corresponding circuit elements being designated by corresponding'reference numerals, except that the s ource of space current 3I is omitted for simplicity, the resistors 29 and 33 are lumped into a common, cathode resistor K, and

the source of the signals to be amplified is represented simply by an alternating current source eo. 'Ihe selectable portion of the load resistor 21 below the tap 36 is designated as R.

'Ihe magnitude of the signal voltage ei applied to the input electrodes of vacuum tube I9 is given by the following equation:

ei=eo-eigmK'-e2gmK where: gm=transconductances of the vacuum tubes I9 and 20, which in this analysis are assumed to be equal, ei=the signal voltage applied between the cathode and grid of tube I9, i ez=the signal voltage applied between the cathode and grid of tube 20. i K=the total resistance of the resistors 29 and 33.

The magnitude of the signal voltage applied to the input electrodes of vacuum tube 20 is given by the following equation:

ez=e1gmKe1ymRe2gmK (q2) 'I'he solution of Equations-1 and 2when simplified,

e2 1'+2g.K-,2RKa For balanced output, the voltages e1 and en should be equal; that is, the ratio of en to e1 should b unity. Thus, from Equations 3 and 4:

l 3 Further, it is desired that the output of the amplifier shall be balanced over a wide range of values of the portion of the signal output of vacuum tube I9 which is applied to the input circuit of vacuum tube 20, that is, over a wide range o f values of R. Thus, when R=0, Equation 5a for'balanced output becomes:

' satised if gmK is large compared to unity. If

gmK has such value, Equation 5a simplies into the following form:

-It will now be evident from Equation 7 that the amplifier has a substantially balanced signal output over a wide range of values of R if K is proportioned to be large compared to R.

From the above mathematical analysis of the Fig. 1 amplier arrangement, it will be evident that, the only conditions which must be satied to effect a substantially balanced output of the amplifier I6 over a Wide range of values of R are;

first, gmK must be large in comparison to unity as the axis of ordinates and gmR' as the axis of abscissae for several values of gmK. The brokenline curve A represents the ratio of the input voltages to the vacuum tubes I9 and 20 when the value of the resistor 33 is zero and it will be seen from this curve that the ratio of input voltages is unity, whereby the output of the amplier is balanced, only at one value of R. However, when the resistor 33 has a value only suiiciently large that the product ymK equals 9, the ratio of the input voltage tubes I9 and 20 is representedby curve -B and it Will be evident that the output of the ampliiier I6 is substantially balanced over a wide range of values of R. Curve C represents .the condition when the `value of the product gmK is very large compared to unity.

Adjustment of the tap 36 on the load resistor 21 provides an adjustment of the gain or repeating ratio of the amplier IB. This will be evident from Equations 3 and '4 which simplify int the following forms when the resistor K is proportioned in the manner above described;

It will be seen from Equations 8`and 9 that the voltage applied to the input electrodes of tubes I9 and 20 has a value one-half that of the input signal voltage eo when the resistor R has a value oscillatory for values of gmR greater than 2. Thus, the repeating ratio or gain of the amplier I 6 may be controlled over a wide range of values by adjustment of the tap 36 on the load resistor 21.

'I'his variation of gain of the amplifier I6 with adjustment of the tap 36 is graphically shown by the curves of Fig. 4 in which Equations 3 and 4 are plotted with the ratio of the input voltage applied to either the tube I9 or the tube 20 to the voltage applied to amplifier I6 as the axis of ordinates and the product gmR as the axis of abscissae. The solid-line curve D corresponds to Equation 3 and the broken-line curve E to Equation 4, the product gmK being given the value of 9 in each instance. It will begevident from these curves that the amplier I6 is` degenerative for values of gmR less than unity and regenerative for largervalues of gmR, the amplifier becoming self-oscillatory when the product gmR has a value equal to or greater than 2.

The inherent capacitance between the cathodes of tubesv I9 and 20 and ground may, in certain instances, as when the frequency of the signal applied to amplifier I6 is relatively high, eiect a slight unbaiance of the output of the amplifier.

The effect of such lcapacitance is avoided in the f rangement remains balanced over a Wide range arrangement of Fig. 5 whichis a circuit diagram representing a modied form of the invention es- Ientially similar to that of Fig. 1, similar' circuit .-Ieme'nts being designated by similar reference numerals, except that an impedance comprising al resistor 33' is included in a circuit portion common to the output circuits of vacuum tubes additionally inserted between the control grid 22 and the condenser 35 in order that equal poirtions of the signal potential developed across the resistor 33' shall be applied tothe grids 2l and 22 of the respective vacuum tubes I9 and 20. That is, the resistor 38 and the grid resistor 28 are effectively a voltage divider and comprise means for applying to the input circuit of tube I9 at least a predetermined portion of the signai voltage developed across the resistor 33. The resistor 39 and the grid'resistor 30 similarly are eiectively a voltage divider and comprise means vfor applying at least the same predetermined portion of the signal voltage to the input circuit of vacuum tube 20. Ilhe screen grids of tubes I9 and 20 are energized"from the battery 3l through tubes I9 and 20. That is, a proportionate part,A

preferably all, of the signal potentials developed. across the resistor 33 is applied to the screen electrodes or grids of tubes I9 and 20 which may in certain cases thus exert an even stronger balancing action than do thecontrol grids 2I and 22 ofI these tubes which have applied thereto only a portion of the signal voltages developed across the resistor 33 by virtue of the voltage divisionv effected by the resistors 38, 28 and 39, 30. It can be shown that the outputv of the Fig. 5 arpart of the signaipotential developed across the impedance 33' which is applied to the screen grids of tubes I9 and 20, and gm is the transconductances of the screen grid electrodes of the tubes I9 and 20; and, second, that the value of the resistor K be large in comparison tothe value of the resistor R, the parameters of these relations having the same signicance as in. Equations 1 to 9'except that K represents-the value of the resistor 33' alonev and doesnot include the resistor 29 as did the resistor Kv in i the analysis of the Figs. 1 and 2 arrangements.

While it has been statedthat the impedance 33 in Fig. 1 and 33 in Fig. 5 comprises a resistor, it will be evident that other than resistive elements may be employed provided that the impedance has a primarily resistive component over the band of frequencies of the signal to b e-translated by the signal amplifier. The input and output circuits of the vacuum tubes of the amplifier may have a predominant reactive component without affecting the desired balanced operation.y

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skiliedin the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore,

aimed in the appended claims to vcover all suchchanges and modiiicatlons as fall within th' true spirit and scope of the invention.

What is claimed is: 1. A signal amplifier comprising, a pair o vacuum tubes each having input electrodes and at least a portion of the resistor 33', and preferl ably all thereof, and there is thus applied to the and will not be repeated. The desired balance of amplier output is greatly improved in this arrangement by the action of the screen grids of an associated input 'circuit and output electrodes and an associated output circuit, said output circuits being connected to form a balanced amplier output circuit, meansfor applying to the input circuit of one of said vacuum tubes a signal to be amplied, means for applying to the input circuit of the other said vacuum tubes a portion of the output of said one vacuum tube, and means for maintaining the output of said ampli` fier balanced over a. range of values of said portion applied to the input circuit of said other vacuum tube comprising a relatively high impedance eiectively common to all of said input and output circuits, the value of said impedance being such that the product of said value and the mutual conductance of one of said vacuum tubes is a number considerably greater than unity.

2. A signal amplifier comprising, a pair of vacuum tubes each having input electrodes and an associated input circuit and output electrodes and an associated output circuit, said vacuum tubes having a cathode circuit common to said output circuits and said output circuits being connected to form a balanced amplifier output circuit, means for applying to the input circuit of one of said vacuum tubes a signal to be amplified, means for applying to the input circuit of the other of said vacuum tubes a portion of the output of said one vacuum tube, and means for maintaining the output of said amplifier balanced over a range of values of said portion applied to the input circuit of said other vacuum tube comprising a relatively high irnpedance included in said common cathode circuit and effectively common to all of said input and output circuits, the value of said impedance being such that the product of said value and the mutual conductance of one of said vacuum tubes is a number considerably greater than unity.

3'. A signal ampliiier comprising, a pair of vacuum tubes each having input electrodes and an associated input circuit and output electrodes and an .associated output circuit, said output circuits being connected to form a balanced amplifier output circuit, means for applying to the input circuit of one of said vacuum tubes a signal to be amplified, means for applying to the input circuit of the other ofsaid vacuum tubes a portion of the output of said onevacuum tube, and means formaintaining the output of said ampliiier balanced over a range of values of said portion applied to the input circuit of said other vacuum tube comprising a resistor of relatively large value eil'ectively common to all of said input and output circuits,

the value of said resistor being such that the product of said value and the mutual conductance of one of said vacuum tubes is a numbe considerably greater than unity. l

4. A signal amplifier compris'ng, a pair of vacuum tubes each having input lectrodes `and an associated input circuit and output electrodes and an associated output circuit', said output circuits having a common circuit portion and being connected to form a balanced amplifier circuit, means for applying to the input circuit of one of said vacuum tubes a signal to be amplified, means for applying to the input circuit of the other of said vacuum tubes a portion of the output of said one vacuum tube, and means for maintaining the output of said amplifier balancedvovera range of values of said portion applied to the input circuit of said other vacuum tube comprising a relatively high impedance included in said common circuit portion, the value of said impedance being such that the product of said value and the mutual conductance of one of said vacuum tubes is a number considerably greater than unity. and means for applying to each of 'said input circuits at least a portion of the signal potentia developed across said impedance.

5. A signal amplifier comprising, a pair of vacuum tubes each having input electrodes and en associated input circuit and output electrodes and an associated output circuit, said `output circuits having a common circuit portion and being connected to form a balancedamplifier circuit, means for applying to the input circuit of one of said vacuum tubes a signal to be amplified, means for applying to the input circuit of the other of said vacuum tubes a portion of the output of said one vacuum tube, and l means for maintaining the output of said amplifier balanced over a range of values of said portion applied to the input circuit of said-other vacuum tube comprising a resistor of relatively large value included in said common circuit portion, the value of said resistor being such that the product of -said value and the mutual conductance of one of said vacuum tubes is a number considerably greater than unity, and means for applying to each of said input circuits at least a portion of the signal potential developed across said resistor.

6. A signal amplifier comprising, a pair of vacuum tubes each having a screen electrode and having input electrodes and an associated input circuit and output electrodes and an associated output circuit, said output circuits having a common circuit portion and being connected to form a balanced amplifier circuit, means for applying to the input circuit of one of said vacuum tubes a signal to be amplified, means for applying to the input circuit of the other of said vacuum tubes a portion of the output of said one vacuum tube, and means for maintaining the output of said amplier balanced over a range of values of said portion applied to the input circuit of said other vacuum tube comprisinga relatively high impedance included in said common circuit portion, the value of said impedance being such that the product of said value and the mutual conductanceY of one of said vacuum tubes is a number considerably greater than unity, .and means for applying to said screen electrodes at least a portion of the signal potentials developed across said impedance.

'7. A signal amplifier comprising, a pair o .i

vacuum tubes each having input electrodes and an associated input circuit and output electrodes 4and an associated output circuit, said output circuits being connected to form a balanced amplifier output circuit, means for applying to the input circuit of one of said vacuum tubes a signal to beampliiied, means for applying to the input circuit of the other of said vacuum tubes a portion of the output of said one vacuum tube, means for maintaining the output of said amplifler balanced over a range of values of said portion applied to the input circuit of said other vacuum tube comprising a relatively high impedance effectively common to all of said input and output circuits, the value of said impedance Abeing such that the product of said value and unity, and means for selectively controlling the value of said portion applied to the input circuit of said other vacuum tube to control the repeating ratio of said amplifier.

8. A signal amplifier comprising, a pair of vacuum tubes each having input electrodes and an associated input circuit and output electrodes and an associated output circuit, said output circuits being connected to form a balanced amplifier output circuit, means for applying to'the input circuit of one of saidvacuum tubes a signal to be amplied, the output circuit of said one vacuum tube includinga load resistor, means for applying to the input circuit of the other of said vacuum tubes a portion of the output of said one vacuum tube, means for maintaining the output of said amplifier balanced over a range of values of said portion applied to the input circuit of said other vacuum tube comprising a 9. A signal amplier comprising, a pair of vacuum tubes each having transconductance gm and having input electrodes and an associated input circuit and output electrodes and an associated output circuit,` said output circuits being connected to form a balanced amplier output circuit, means for applying to the input circuit of one of said .vacuum tubes a signal to be amplied, the output circuit of said one vacuum tube including a load impedance, means' for coupling the input circuit of the other of said vacuum tubes to a selectable portion R of said load impedance, and means for maintaining the output of said amplifier balanced over a range of values of said portion R coupled to the input circuit of said other vacuum tube comprising a relatively high impedance K effectively common to all of said input and output circuits, the value of said impedance K being so proportioned that it is large compared to any value of R in said range of values and the product gmK is large compared to unity.

10. AV signal amplier comprising, a pair of vacuum tubes each having transconductance gm and having input electrodes and an associated input circuit and output electrodes and an associatedvoutput circuit, said output circuits having a common circuit portion and being connected to form a balanced amplifier circuit, meansl for applying to the input circuit of one of said vacuum tubes a signal to be amplied,

the output circuit of said one vacuum tube including a load impedance, means for coupling the input clrcu'it of the other of said vacuum tubes to a selectable portion R of said load impedance, and means for maintaining the output of said amplifier balanced over a range of values of said portion R coupled to the input circuit of said other vacuum tube comprising a relatively high impedance K included in said common circuit portion, and means for applying to the inputcircuits of both of said vacuum tubes a proi portionate part 1c of the signal potential de- Veloped across said high impedance, the value of said impedance K being so proportioned that,

it is large compared to any value of R in said range of values and the product gmkK is large vcompared to unity.

ance, means for coupling the input circuit of the other of said vacuum tubes to a selectable portion R of said load impedance, and means for.

maintaining the output of said Aamplifierbalanced over a range of values of said portion R coupled to the input circuit of said other vacuum tube comprising a relatively high impedance K included in said common circuit portion, and means for applying to said screen electioned that it is large compared to any value of R in said range of values and the product g'mIcK is large compared to unity.

HAROLD M. LEWIS. 

